Add test to validate encoder command builder output

[cdgriffith]

The Whole UI needs test, but I think most important is to make sure with whatever input is supplied to the command builders fastflix/encoders/<name>/command_builder.py and the general functions in fastflix/encoders/common/helpers.py is the most important to get tests for first.

I have some minimal start on pytests, but would appreciate any help in this effort.